In conventional powder coating methods and apparatus, a pressurized electrostatically charged mixture of gas and powder particles is sprayed outwardly from a gun in the direction of an object to be coated. The charged powder particles in the mixture repel one another as they travel toward the object to be coated, and the lower electrical potential of the object to be coated electrostatically attracts the particles.
When powder coating an object having a large surface area using a single spray device the spray device or the object must be moved so as to coat the whole area, and this takes a considerable time and reduces the maximum throughput of a powder coating system.
Conventional electrostatic powder coating methods and apparatus present further problems when coating a large surface area. When using an external electrode to establish an electrostatic field between the spray device, or gun, and the object, the strongest lines of electrostatic force are located along the direct line between the electrode and the nearest point on the object (the `Faraday cage` effect), and electrostatic lines of force directed towards the more distant portions of the surface to be coated will be significantly weaker, with the result that the object is coated unevenly, with a varying depth of powder since the charged powder follows the electrostatic field lines to the object being coated. Additionally, particles having a higher electrostatic charge retain their charge after deposition, thereby repulsing subsequent particles and resulting in uneven coating.
These problems are addressed to an extent in International Patent Application No. PCT/GB94/01991 (publication No. WO 95/08397), which discloses a powder spray coating apparatus effective to produce substantially flat, thin spray pattern in the form of a spray disc. The apparatus is reciprocated, thereby reciprocating the spray pattern, so as to provide a uniform powder coating over a large surface area. The apparatus comprises means for electrostatically charging the powder particles before they are discharged from the spray device; there is therefore no requirement for external charging electrodes, and thus this cause of non uniformity of coating is substantially eliminated.
Apparatus of the type disclosed in PCT/GB94/01991 is particularly suitable for coating objects such as the interior of an open-ended drum. However, whatever the object to be coated, it is necessary to provide relative motion between the object and the spray device in order to coat a large surface area, because the apparatus of PCT/GB94/01991 produces a thin flat spray pattern, in the shape of a disc.